Yahoo! Mail and Message Threads?

I keep trying out the new Yahoo! Mail and the one feature that makes it unworkable for me is broken headers.

Specifically, support for message threads (http://people.dsv.su.se/~jpalme/ietf/message-threading.html) doesn’t exist, but it does in Yahoo! Mail classic. The reason this is problematic for me is that I am on a number of technical mailing lists and people complain that the new Yahoo! Mail’s broken headers break the list thread view in their email clients.

I’ve submitted feedback to Yahoo! about this (a couple of times, actually), but I’ve never heard anything back. Can anyone shed any light on why they’ve never fixed this?

Cheers,
Ovid